One bit that is still a bit confusing to me in your use case is if you need to take a snapshot right after some event in your application. Even if you're able to tell ZooKeeper to take a snapshot, there is no guarantee that it will happen at the exact point you want it if update operations keep coming.

If you use your four-letter word approach, then would you search for the leader or would you simply take a snapshot at any server? If it has to go through the leader so that you make sure to have the most recent committed state, then it might not be a bad idea to have an api call that tells the leader to take a snapshot at some directory of your choice. Informing you the name of the snapshot file so that you can copy sounds like an option, but perhaps it is not as convenient.

The approach of adding another server is not very clear. How do you force it to be the leader? Keep in mind that if it crashes, then it will lose leadership.

> It looks like the "dev" mailing list is rather inactive. Over the past few> days I only saw several automated emails from JIRA and this is pretty much> it. Contrary to this, the "user" mailing list seems to be more alive and> more populated.> > With this in mind, please allow me to cross-post here the message I sent> into the "dev" list a few days ago.> > > Regards,> /Sergey> > === forwarded message begins here ==> > Hi!> > I'm facing the problem that has been raised by multiple people but none of> the discussion threads seem to provide a good answer. I dug in Zookeeper> source code trying to come up with some possible approaches and I would> like to get your inputs on those.> > Initial conditions:> > * I have an ensemble of five Zookeeper servers running v3.4.5 code.> * The size of a committed snapshot file is in vicinity of 1GB.> * There are about 80 clients connected to the ensemble.> * Clients a heavily read biased, i.e., they mostly read and rarely write. I> would say less than 0.1% of queries modify the data.> > Problem statement:> > * Under certain conditions, I may need to revert the data stored in the> ensemble to an earlier state. For example, one of the clients may ruin the> application-level data integrity and I need to perform a disaster recovery.> > Things look nice and easy if I'm dealing with a single Zookeeper server. A> file-level copy of the data and dataLog directories should allow me to> recover later by stopping Zookeeper, swapping the corrupted data and> dataLog directories with a backup, and firing Zookeeper back up.> > Now, the ensemble deployment and the leader election algorithm in the> quorum make things much more difficult. In order to restore from a single> file-level backup, I need to take the whole ensemble down, wipe out data> and dataLog directories on all servers, replace these directories with> backed up content on one of the servers, bring this server up first, and> then bring up the rest of the ensemble. This [somewhat] guarantees that the> populated Zookeeper server becomes a member of a majority and populates the> ensemble. This approach works but it is very involving and, thus,> error-prone due to a human error.> > Based on a study of Zookeeper source code, I am considering the following> alternatives. And I seek advice from Zookeeper development community as to> which approach looks more promising or if there is a better way.> > Approach #1:> > Develop a complementary pair of utilities for export and import of the> data. Both utilities will act as Zookeeper clients and use the existing> API. The "export" utility will recursively retrieve data and store it in a> file. The "import" utility will first purge all data from the ensemble and> then reload it from the file.> > This approach seems to be the simplest and there are similar tools> developed already. For example, the Guano Project:> https://github.com/d2fn/guano

Thank you for your response, Flavio. I apologize, I did not provide a clearexplanation of the use case.

This backup/restore is not intended to be tied to any write event, instead,it is expected to run as a periodic (daily?) cron job on one of theservers, which is not guaranteed to be the leader of the ensemble. There isno expectation that all recent changes are committed and persisted to disk.The system can sustain the loss of several hours worth of recent changes inthe event of restore.

As for finding the leader dynamically and performing backup on it, thisapproach could be more difficult as the leader can change time to time andI still need to fetch the file to store it in my designated backuplocation. Taking backup on one server and picking it up from a local filesystem looks less error-prone. Even if I went the fancy route and hadZookeeper send me the serialized DataTree in response to the 4wl, thisapproach would involve a lot of moving parts.

I have already made a PoC for a new 4wl that invokes takeSnapshot() andreturns an absolute path to the snapshot it drops on disk. I have alreadyprotected takeSnapshot() from concurrent invocation, which is likely tocorrupt the snapshot file on disk. This approach works but I'm thinking totake it one step further by providing the desired path name as an argumentto my new 4lw and to have Zookeeper server drop the snapshot into thespecified file and report success/failure back. This way I can avoidcluttering the data directory and interfering with what Zookeeper findswhen it scans the data directory.

Approach with having an additional server that would take the leadershipand populate the ensemble is just a theory. I don't see a clean way ofmaking a quorum member the leader of the quorum. Am I overlooking somethingsimple?

In backup and restore of an ensemble the biggest unknown for me remainspopulating the ensemble with desired data. I can think of two ways:

1. Clear out all servers by stopping them, purge version-2 directories,restore a snapshot file on one server that will be brought first, and thenbring up the rest of the ensemble. This way I somewhat force the firstserver to be the leader because it has data and it will be the only memberof a quorum with data, provided to the way I start the ensemble. This lookslike a hack, though.

2. Clear out the ensemble and reload it with a dedicated client using theprovided Zookeeper API.

With the approach of backing up an actual snapshot file, option #1 appearsto be more practical.

I wish I could start the ensemble with a designate leader that wouldbootstrap the ensemble with data and then the ensemble would go into itsnormal business...

> One bit that is still a bit confusing to me in your use case is if you> need to take a snapshot right after some event in your application. Even if> you're able to tell ZooKeeper to take a snapshot, there is no guarantee> that it will happen at the exact point you want it if update operations> keep coming.>> If you use your four-letter word approach, then would you search for the> leader or would you simply take a snapshot at any server? If it has to go> through the leader so that you make sure to have the most recent committed> state, then it might not be a bad idea to have an api call that tells the> leader to take a snapshot at some directory of your choice. Informing you> the name of the snapshot file so that you can copy sounds like an option,> but perhaps it is not as convenient.>> The approach of adding another server is not very clear. How do you force> it to be the leader? Keep in mind that if it crashes, then it will lose> leadership.>> -Flavio>> On Jul 8, 2013, at 8:34 AM, Sergey Maslyakov <[EMAIL PROTECTED]> wrote:>> > It looks like the "dev" mailing list is rather inactive. Over the past> few> > days I only saw several automated emails from JIRA and this is pretty

Just saw that this is the corresponding use case to the question postedin dev list.

In order to restore the data to a given point in time correctly, you needboth snapshot and txnlog. This is because zookeeper snapshot is fuzzy andsnapshot alone may not represent a valid state of the server if there arein-flight requests.

The 4wl command should cause the server to roll the log and take asnapshot similar to periodic snapshotting operation. Your backup scriptneed grap the snapshot and corresponding txnlog file from the data dir.

To restore, just shutdown all hosts, clear the data dir, copy over thesnapshot and txnlog, and restart them.-- Thawan Kooburat

On 7/8/13 3:28 PM, "Sergey Maslyakov" <[EMAIL PROTECTED]> wrote:

>Thank you for your response, Flavio. I apologize, I did not provide a>clear>explanation of the use case.>>This backup/restore is not intended to be tied to any write event,>instead,>it is expected to run as a periodic (daily?) cron job on one of the>servers, which is not guaranteed to be the leader of the ensemble. There>is>no expectation that all recent changes are committed and persisted to>disk.>The system can sustain the loss of several hours worth of recent changes>in>the event of restore.>>As for finding the leader dynamically and performing backup on it, this>approach could be more difficult as the leader can change time to time and>I still need to fetch the file to store it in my designated backup>location. Taking backup on one server and picking it up from a local file>system looks less error-prone. Even if I went the fancy route and had>Zookeeper send me the serialized DataTree in response to the 4wl, this>approach would involve a lot of moving parts.>>I have already made a PoC for a new 4wl that invokes takeSnapshot() and>returns an absolute path to the snapshot it drops on disk. I have already>protected takeSnapshot() from concurrent invocation, which is likely to>corrupt the snapshot file on disk. This approach works but I'm thinking to>take it one step further by providing the desired path name as an argument>to my new 4lw and to have Zookeeper server drop the snapshot into the>specified file and report success/failure back. This way I can avoid>cluttering the data directory and interfering with what Zookeeper finds>when it scans the data directory.>>Approach with having an additional server that would take the leadership>and populate the ensemble is just a theory. I don't see a clean way of>making a quorum member the leader of the quorum. Am I overlooking>something>simple?>>In backup and restore of an ensemble the biggest unknown for me remains>populating the ensemble with desired data. I can think of two ways:>>1. Clear out all servers by stopping them, purge version-2 directories,>restore a snapshot file on one server that will be brought first, and then>bring up the rest of the ensemble. This way I somewhat force the first>server to be the leader because it has data and it will be the only member>of a quorum with data, provided to the way I start the ensemble. This>looks>like a hack, though.>>2. Clear out the ensemble and reload it with a dedicated client using the>provided Zookeeper API.>>With the approach of backing up an actual snapshot file, option #1 appears>to be more practical.>>I wish I could start the ensemble with a designate leader that would>bootstrap the ensemble with data and then the ensemble would go into its>normal business...>>>>On Mon, Jul 8, 2013 at 4:30 PM, Flavio Junqueira><[EMAIL PROTECTED]>wrote:>>> One bit that is still a bit confusing to me in your use case is if you>> need to take a snapshot right after some event in your application.>>Even if>> you're able to tell ZooKeeper to take a snapshot, there is no guarantee>> that it will happen at the exact point you want it if update operations>> keep coming.>>>> If you use your four-letter word approach, then would you search for the

I think what we are looking at is a point in time restore functionality.How about adding a feature that says go back to a specific zxid/timestamp.This way before doing any change to zookeeper simply note down thetimestamp/zxid on leader. If things go wrong after making changes, bringdown zookeepers and provide additional parameter of a zxid/timestamp whilerestarting. The server can go the exact point and make it current. Thefollowers can be started blank.

> Just saw that this is the corresponding use case to the question posted> in dev list.>> In order to restore the data to a given point in time correctly, you need> both snapshot and txnlog. This is because zookeeper snapshot is fuzzy and> snapshot alone may not represent a valid state of the server if there are> in-flight requests.>> The 4wl command should cause the server to roll the log and take a> snapshot similar to periodic snapshotting operation. Your backup script> need grap the snapshot and corresponding txnlog file from the data dir.>> To restore, just shutdown all hosts, clear the data dir, copy over the> snapshot and txnlog, and restart them.>>> --> Thawan Kooburat>>>>>> On 7/8/13 3:28 PM, "Sergey Maslyakov" <[EMAIL PROTECTED]> wrote:>> >Thank you for your response, Flavio. I apologize, I did not provide a> >clear> >explanation of the use case.> >> >This backup/restore is not intended to be tied to any write event,> >instead,> >it is expected to run as a periodic (daily?) cron job on one of the> >servers, which is not guaranteed to be the leader of the ensemble. There> >is> >no expectation that all recent changes are committed and persisted to> >disk.> >The system can sustain the loss of several hours worth of recent changes> >in> >the event of restore.> >> >As for finding the leader dynamically and performing backup on it, this> >approach could be more difficult as the leader can change time to time and> >I still need to fetch the file to store it in my designated backup> >location. Taking backup on one server and picking it up from a local file> >system looks less error-prone. Even if I went the fancy route and had> >Zookeeper send me the serialized DataTree in response to the 4wl, this> >approach would involve a lot of moving parts.> >> >I have already made a PoC for a new 4wl that invokes takeSnapshot() and> >returns an absolute path to the snapshot it drops on disk. I have already> >protected takeSnapshot() from concurrent invocation, which is likely to> >corrupt the snapshot file on disk. This approach works but I'm thinking to> >take it one step further by providing the desired path name as an argument> >to my new 4lw and to have Zookeeper server drop the snapshot into the> >specified file and report success/failure back. This way I can avoid> >cluttering the data directory and interfering with what Zookeeper finds> >when it scans the data directory.> >> >Approach with having an additional server that would take the leadership> >and populate the ensemble is just a theory. I don't see a clean way of> >making a quorum member the leader of the quorum. Am I overlooking> >something> >simple?> >> >In backup and restore of an ensemble the biggest unknown for me remains> >populating the ensemble with desired data. I can think of two ways:> >> >1. Clear out all servers by stopping them, purge version-2 directories,> >restore a snapshot file on one server that will be brought first, and then> >bring up the rest of the ensemble. This way I somewhat force the first> >server to be the leader because it has data and it will be the only member> >of a quorum with data, provided to the way I start the ensemble. This> >looks> >like a hack, though.> >> >2. Clear out the ensemble and reload it with a dedicated client using the> >provided Zookeeper API.> >> >With the approach of backing up an actual snapshot file, option #1 appears

On restore part, I think having a separate utility to manipulate thedata/snap dir (by truncating the log/removing snapshot to a given zxid)would be easier than modifying the server.-- Thawan Kooburat

On 7/8/13 6:34 PM, "kishore g" <[EMAIL PROTECTED]> wrote:

>I think what we are looking at is a point in time restore functionality.>How about adding a feature that says go back to a specific zxid/timestamp.>This way before doing any change to zookeeper simply note down the>timestamp/zxid on leader. If things go wrong after making changes, bring>down zookeepers and provide additional parameter of a zxid/timestamp while>restarting. The server can go the exact point and make it current. The>followers can be started blank.>>>>On Mon, Jul 8, 2013 at 5:53 PM, Thawan Kooburat <[EMAIL PROTECTED]> wrote:>>> Just saw that this is the corresponding use case to the question posted>> in dev list.>>>> In order to restore the data to a given point in time correctly, you>>need>> both snapshot and txnlog. This is because zookeeper snapshot is fuzzy>>and>> snapshot alone may not represent a valid state of the server if there>>are>> in-flight requests.>>>> The 4wl command should cause the server to roll the log and take a>> snapshot similar to periodic snapshotting operation. Your backup script>> need grap the snapshot and corresponding txnlog file from the data dir.>>>> To restore, just shutdown all hosts, clear the data dir, copy over the>> snapshot and txnlog, and restart them.>>>>>> -->> Thawan Kooburat>>>>>>>>>>>> On 7/8/13 3:28 PM, "Sergey Maslyakov" <[EMAIL PROTECTED]> wrote:>>>> >Thank you for your response, Flavio. I apologize, I did not provide a>> >clear>> >explanation of the use case.>> >>> >This backup/restore is not intended to be tied to any write event,>> >instead,>> >it is expected to run as a periodic (daily?) cron job on one of the>> >servers, which is not guaranteed to be the leader of the ensemble.>>There>> >is>> >no expectation that all recent changes are committed and persisted to>> >disk.>> >The system can sustain the loss of several hours worth of recent>>changes>> >in>> >the event of restore.>> >>> >As for finding the leader dynamically and performing backup on it, this>> >approach could be more difficult as the leader can change time to time>>and>> >I still need to fetch the file to store it in my designated backup>> >location. Taking backup on one server and picking it up from a local>>file>> >system looks less error-prone. Even if I went the fancy route and had>> >Zookeeper send me the serialized DataTree in response to the 4wl, this>> >approach would involve a lot of moving parts.>> >>> >I have already made a PoC for a new 4wl that invokes takeSnapshot() and>> >returns an absolute path to the snapshot it drops on disk. I have>>already>> >protected takeSnapshot() from concurrent invocation, which is likely to>> >corrupt the snapshot file on disk. This approach works but I'm>>thinking to>> >take it one step further by providing the desired path name as an>>argument>> >to my new 4lw and to have Zookeeper server drop the snapshot into the>> >specified file and report success/failure back. This way I can avoid>> >cluttering the data directory and interfering with what Zookeeper finds>> >when it scans the data directory.>> >>> >Approach with having an additional server that would take the>>leadership>> >and populate the ensemble is just a theory. I don't see a clean way of>> >making a quorum member the leader of the quorum. Am I overlooking>> >something>> >simple?>> >>> >In backup and restore of an ensemble the biggest unknown for me remains>> >populating the ensemble with desired data. I can think of two ways:>> >>> >1. Clear out all servers by stopping them, purge version-2 directories,>> >restore a snapshot file on one server that will be brought first, and>>then>> >bring up the rest of the ensemble. This way I somewhat force the first

Agree, we already have such a tool. In fact we use it to reconstruct thesequence of events that led to a failure and actually restore the system toa previous stable point and replay the events. Unfortunately this is tiedclosely with Helix but it should be easy to make this a generic tool.

> On restore part, I think having a separate utility to manipulate the> data/snap dir (by truncating the log/removing snapshot to a given zxid)> would be easier than modifying the server.>>> --> Thawan Kooburat>>>>>> On 7/8/13 6:34 PM, "kishore g" <[EMAIL PROTECTED]> wrote:>> >I think what we are looking at is a point in time restore functionality.> >How about adding a feature that says go back to a specific zxid/timestamp.> >This way before doing any change to zookeeper simply note down the> >timestamp/zxid on leader. If things go wrong after making changes, bring> >down zookeepers and provide additional parameter of a zxid/timestamp while> >restarting. The server can go the exact point and make it current. The> >followers can be started blank.> >> >> >> >On Mon, Jul 8, 2013 at 5:53 PM, Thawan Kooburat <[EMAIL PROTECTED]> wrote:> >> >> Just saw that this is the corresponding use case to the question posted> >> in dev list.> >>> >> In order to restore the data to a given point in time correctly, you> >>need> >> both snapshot and txnlog. This is because zookeeper snapshot is fuzzy> >>and> >> snapshot alone may not represent a valid state of the server if there> >>are> >> in-flight requests.> >>> >> The 4wl command should cause the server to roll the log and take a> >> snapshot similar to periodic snapshotting operation. Your backup script> >> need grap the snapshot and corresponding txnlog file from the data dir.> >>> >> To restore, just shutdown all hosts, clear the data dir, copy over the> >> snapshot and txnlog, and restart them.> >>> >>> >> --> >> Thawan Kooburat> >>> >>> >>> >>> >>> >> On 7/8/13 3:28 PM, "Sergey Maslyakov" <[EMAIL PROTECTED]> wrote:> >>> >> >Thank you for your response, Flavio. I apologize, I did not provide a> >> >clear> >> >explanation of the use case.> >> >> >> >This backup/restore is not intended to be tied to any write event,> >> >instead,> >> >it is expected to run as a periodic (daily?) cron job on one of the> >> >servers, which is not guaranteed to be the leader of the ensemble.> >>There> >> >is> >> >no expectation that all recent changes are committed and persisted to> >> >disk.> >> >The system can sustain the loss of several hours worth of recent> >>changes> >> >in> >> >the event of restore.> >> >> >> >As for finding the leader dynamically and performing backup on it, this> >> >approach could be more difficult as the leader can change time to time> >>and> >> >I still need to fetch the file to store it in my designated backup> >> >location. Taking backup on one server and picking it up from a local> >>file> >> >system looks less error-prone. Even if I went the fancy route and had> >> >Zookeeper send me the serialized DataTree in response to the 4wl, this> >> >approach would involve a lot of moving parts.> >> >> >> >I have already made a PoC for a new 4wl that invokes takeSnapshot() and> >> >returns an absolute path to the snapshot it drops on disk. I have> >>already> >> >protected takeSnapshot() from concurrent invocation, which is likely to> >> >corrupt the snapshot file on disk. This approach works but I'm> >>thinking to> >> >take it one step further by providing the desired path name as an> >>argument> >> >to my new 4lw and to have Zookeeper server drop the snapshot into the> >> >specified file and report success/failure back. This way I can avoid> >> >cluttering the data directory and interfering with what Zookeeper finds> >> >when it scans the data directory.> >>

This sounds like a very elaborate tool. I was trying to find a simplisticapproach but what Thawan said about "fuzzy snapshots" makes me a littleafraid that there is no simple solution.On Mon, Jul 8, 2013 at 11:05 PM, kishore g <[EMAIL PROTECTED]> wrote:

> Agree, we already have such a tool. In fact we use it to reconstruct the> sequence of events that led to a failure and actually restore the system to> a previous stable point and replay the events. Unfortunately this is tied> closely with Helix but it should be easy to make this a generic tool.>> Sergey is this something that will be useful in your case.>> Thanks,> Kishore G>>> On Mon, Jul 8, 2013 at 8:09 PM, Thawan Kooburat <[EMAIL PROTECTED]> wrote:>> > On restore part, I think having a separate utility to manipulate the> > data/snap dir (by truncating the log/removing snapshot to a given zxid)> > would be easier than modifying the server.> >> >> > --> > Thawan Kooburat> >> >> >> >> >> > On 7/8/13 6:34 PM, "kishore g" <[EMAIL PROTECTED]> wrote:> >> > >I think what we are looking at is a point in time restore> functionality.> > >How about adding a feature that says go back to a specific> zxid/timestamp.> > >This way before doing any change to zookeeper simply note down the> > >timestamp/zxid on leader. If things go wrong after making changes, bring> > >down zookeepers and provide additional parameter of a zxid/timestamp> while> > >restarting. The server can go the exact point and make it current. The> > >followers can be started blank.> > >> > >> > >> > >On Mon, Jul 8, 2013 at 5:53 PM, Thawan Kooburat <[EMAIL PROTECTED]> wrote:> > >> > >> Just saw that this is the corresponding use case to the question> posted> > >> in dev list.> > >>> > >> In order to restore the data to a given point in time correctly, you> > >>need> > >> both snapshot and txnlog. This is because zookeeper snapshot is fuzzy> > >>and> > >> snapshot alone may not represent a valid state of the server if there> > >>are> > >> in-flight requests.> > >>> > >> The 4wl command should cause the server to roll the log and take a> > >> snapshot similar to periodic snapshotting operation. Your backup> script> > >> need grap the snapshot and corresponding txnlog file from the data> dir.> > >>> > >> To restore, just shutdown all hosts, clear the data dir, copy over the> > >> snapshot and txnlog, and restart them.> > >>> > >>> > >> --> > >> Thawan Kooburat> > >>> > >>> > >>> > >>> > >>> > >> On 7/8/13 3:28 PM, "Sergey Maslyakov" <[EMAIL PROTECTED]> wrote:> > >>> > >> >Thank you for your response, Flavio. I apologize, I did not provide a> > >> >clear> > >> >explanation of the use case.> > >> >> > >> >This backup/restore is not intended to be tied to any write event,> > >> >instead,> > >> >it is expected to run as a periodic (daily?) cron job on one of the> > >> >servers, which is not guaranteed to be the leader of the ensemble.> > >>There> > >> >is> > >> >no expectation that all recent changes are committed and persisted to> > >> >disk.> > >> >The system can sustain the loss of several hours worth of recent> > >>changes> > >> >in> > >> >the event of restore.> > >> >> > >> >As for finding the leader dynamically and performing backup on it,> this> > >> >approach could be more difficult as the leader can change time to> time> > >>and> > >> >I still need to fetch the file to store it in my designated backup> > >> >location. Taking backup on one server and picking it up from a local> > >>file> > >> >system looks less error-prone. Even if I went the fancy route and had> > >> >Zookeeper send me the serialized DataTree in response to the 4wl,> this> > >> >approach would involve a lot of moving parts.> > >> >> > >> >I have already made a PoC for a new 4wl that invokes takeSnapshot()> and> > >> >returns an absolute path to the snapshot it drops on disk. I have> > >>already> > >> >protected takeSnapshot() from concurrent invocation, which is likely

It isn't that bad. The deal is that a snapshot takes time to write to disk. During this time updates are still allowed to the contents of memory. All such updates are logged however so if you have the transaction log from the moment before the snap starts until some moment after the snap completes you can load the snapshot and then replay the log to get a moment in time snapshot as of the time if the final transaction that you have applied.

This works because all if the logged transactions are idem potent. If they are applied to part of the snapshot that already recorded their effect, there is no problem.

If you want you can even do the replay in a side process after the snapshot is complete so that you don't have to carry around the transaction log.

Sent from my iPhone

On Jul 8, 2013, at 21:42, Sergey Maslyakov <[EMAIL PROTECTED]> wrote:

> Kishore,> > This sounds like a very elaborate tool. I was trying to find a simplistic> approach but what Thawan said about "fuzzy snapshots" makes me a little> afraid that there is no simple solution.> > > On Mon, Jul 8, 2013 at 11:05 PM, kishore g <[EMAIL PROTECTED]> wrote:> >> Agree, we already have such a tool. In fact we use it to reconstruct the>> sequence of events that led to a failure and actually restore the system to>> a previous stable point and replay the events. Unfortunately this is tied>> closely with Helix but it should be easy to make this a generic tool.>> >> Sergey is this something that will be useful in your case.>> >> Thanks,>> Kishore G>> >> >> On Mon, Jul 8, 2013 at 8:09 PM, Thawan Kooburat <[EMAIL PROTECTED]> wrote:>> >>> On restore part, I think having a separate utility to manipulate the>>> data/snap dir (by truncating the log/removing snapshot to a given zxid)>>> would be easier than modifying the server.>>> >>> >>> -->>> Thawan Kooburat>>> >>> >>> >>> >>> >>> On 7/8/13 6:34 PM, "kishore g" <[EMAIL PROTECTED]> wrote:>>> >>>> I think what we are looking at is a point in time restore>> functionality.>>>> How about adding a feature that says go back to a specific>> zxid/timestamp.>>>> This way before doing any change to zookeeper simply note down the>>>> timestamp/zxid on leader. If things go wrong after making changes, bring>>>> down zookeepers and provide additional parameter of a zxid/timestamp>> while>>>> restarting. The server can go the exact point and make it current. The>>>> followers can be started blank.>>>> >>>> >>>> >>>> On Mon, Jul 8, 2013 at 5:53 PM, Thawan Kooburat <[EMAIL PROTECTED]> wrote:>>>> >>>>> Just saw that this is the corresponding use case to the question>> posted>>>>> in dev list.>>>>> >>>>> In order to restore the data to a given point in time correctly, you>>>>> need>>>>> both snapshot and txnlog. This is because zookeeper snapshot is fuzzy>>>>> and>>>>> snapshot alone may not represent a valid state of the server if there>>>>> are>>>>> in-flight requests.>>>>> >>>>> The 4wl command should cause the server to roll the log and take a>>>>> snapshot similar to periodic snapshotting operation. Your backup>> script>>>>> need grap the snapshot and corresponding txnlog file from the data>> dir.>>>>> >>>>> To restore, just shutdown all hosts, clear the data dir, copy over the>>>>> snapshot and txnlog, and restart them.>>>>> >>>>> >>>>> -->>>>> Thawan Kooburat>>>>> >>>>> >>>>> >>>>> >>>>> >>>>> On 7/8/13 3:28 PM, "Sergey Maslyakov" <[EMAIL PROTECTED]> wrote:>>>>> >>>>>> Thank you for your response, Flavio. I apologize, I did not provide a>>>>>> clear>>>>>> explanation of the use case.>>>>>> >>>>>> This backup/restore is not intended to be tied to any write event,>>>>>> instead,>>>>>> it is expected to run as a periodic (daily?) cron job on one of the>>>>>> servers, which is not guaranteed to be the leader of the ensemble.>>>>> There>>>>>> is>>>>>> no expectation that all recent changes are committed and persisted to>>>>>> disk.>>>>>> The system can sustain the loss of several hours worth of recent

Its not really elaborate, it is very similar to what zookeeper does when itstarts up. It first reads the latest snapshot file and then the transactionlogs and applies each and every transaction. What I am suggesting is thatinstead of applying all transactions stop at a transaction i provide.

Having this tool will actually simplify your task, you can go back to anypoint in time. Think of a something like this.

checkpoint A // this can store the last zxid or timestamp from the leader.Make changes to zk//if things failsstop zksrollback A//run this on each zk, brings back the cluster to its previousstate.start zks // any order should be fine.Also keep in mind that snapshot is fuzzy only if there are writes happeningwhile taking snapshot. If you are sure no writes will happen when you aretaking the snapshot then you are good. Experts, please correct me if thisis incorrect.

> Its not really elaborate, it is very similar to what zookeeper does when it> starts up. It first reads the latest snapshot file and then the transaction> logs and applies each and every transaction. What I am suggesting is that> instead of applying all transactions stop at a transaction i provide.> > Having this tool will actually simplify your task, you can go back to any> point in time. Think of a something like this.> > checkpoint A // this can store the last zxid or timestamp from the leader.> Make changes to zk> //if things fails> stop zks> rollback A//run this on each zk, brings back the cluster to its previous> state.> start zks // any order should be fine.> > > Also keep in mind that snapshot is fuzzy only if there are writes happening> while taking snapshot. If you are sure no writes will happen when you are> taking the snapshot then you are good. Experts, please correct me if this> is incorrect.If there are no concurrent writes, then the snapshot will contain all zxids up to the one in the file name and that one will be the last. The problem is making sure that there are no concurrent updates... Would you tell all the clients to stop first? Keep trying until you get no concurrent updates? It sounds difficult, right?

-Flavio

> > thanks,> Kishore G> > > On Mon, Jul 8, 2013 at 9:42 PM, Sergey Maslyakov <[EMAIL PROTECTED]> wrote:> >> Kishore,>> >> This sounds like a very elaborate tool. I was trying to find a simplistic>> approach but what Thawan said about "fuzzy snapshots" makes me a little>> afraid that there is no simple solution.>> >> >> On Mon, Jul 8, 2013 at 11:05 PM, kishore g <[EMAIL PROTECTED]> wrote:>> >>> Agree, we already have such a tool. In fact we use it to reconstruct the>>> sequence of events that led to a failure and actually restore the system>> to>>> a previous stable point and replay the events. Unfortunately this is tied>>> closely with Helix but it should be easy to make this a generic tool.>>> >>> Sergey is this something that will be useful in your case.>>> >>> Thanks,>>> Kishore G>>> >>> >>> On Mon, Jul 8, 2013 at 8:09 PM, Thawan Kooburat <[EMAIL PROTECTED]> wrote:>>> >>>> On restore part, I think having a separate utility to manipulate the>>>> data/snap dir (by truncating the log/removing snapshot to a given zxid)>>>> would be easier than modifying the server.>>>> >>>> >>>> -->>>> Thawan Kooburat>>>> >>>> >>>> >>>> >>>> >>>> On 7/8/13 6:34 PM, "kishore g" <[EMAIL PROTECTED]> wrote:>>>> >>>>> I think what we are looking at is a point in time restore>>> functionality.>>>>> How about adding a feature that says go back to a specific>>> zxid/timestamp.>>>>> This way before doing any change to zookeeper simply note down the>>>>> timestamp/zxid on leader. If things go wrong after making changes,>> bring>>>>> down zookeepers and provide additional parameter of a zxid/timestamp>>> while>>>>> restarting. The server can go the exact point and make it current. The>>>>> followers can be started blank.>>>>> >>>>> >>>>> >>>>> On Mon, Jul 8, 2013 at 5:53 PM, Thawan Kooburat <[EMAIL PROTECTED]>>> wrote:>>>>> >>>>>> Just saw that this is the corresponding use case to the question>>> posted>>>>>> in dev list.>>>>>> >>>>>> In order to restore the data to a given point in time correctly, you>>>>>> need>>>>>> both snapshot and txnlog. This is because zookeeper snapshot is>> fuzzy>>>>>> and>>>>>> snapshot alone may not represent a valid state of the server if>> there>>>>>> are>>>>>> in-flight requests.>>>>>> >>>>>> The 4wl command should cause the server to roll the log and take a>>>>>> snapshot similar to periodic snapshotting operation. Your backup>>> script>>>>>> need grap the snapshot and corresponding txnlog file from the data>>> dir.>>>>>> >>>>>> To restore, just shutdown all hosts, clear the data dir, copy over>> the>>>>>> snapshot and txnlog, and restart them.

I think I am having difficulties understanding the "fuzzy" concept. Let'ssay I started to serialize DataTree into a snapshot file and it took 30seconds. During these 30 seconds, the server saw 5 transactions thatupdated the data. Does this mean that the snapshot that I get on disk atthe end of the 30-second interval will have some of these 5 transactions?Or will it have none? Or will it have all of them? Or will it beinconsistent and unreadable by Zookeeper?

For my use case, I am perfectly fine if I get a snapshot with none of these5 transactions, considering that I will pick them up next time I take asnapshot./SergeyOn Tue, Jul 9, 2013 at 12:08 AM, kishore g <[EMAIL PROTECTED]> wrote:

> Its not really elaborate, it is very similar to what zookeeper does when it> starts up. It first reads the latest snapshot file and then the transaction> logs and applies each and every transaction. What I am suggesting is that> instead of applying all transactions stop at a transaction i provide.>> Having this tool will actually simplify your task, you can go back to any> point in time. Think of a something like this.>> checkpoint A // this can store the last zxid or timestamp from the leader.> Make changes to zk> //if things fails> stop zks> rollback A//run this on each zk, brings back the cluster to its previous> state.> start zks // any order should be fine.>>> Also keep in mind that snapshot is fuzzy only if there are writes happening> while taking snapshot. If you are sure no writes will happen when you are> taking the snapshot then you are good. Experts, please correct me if this> is incorrect.>> thanks,> Kishore G>>> On Mon, Jul 8, 2013 at 9:42 PM, Sergey Maslyakov <[EMAIL PROTECTED]>> wrote:>> > Kishore,> >> > This sounds like a very elaborate tool. I was trying to find a simplistic> > approach but what Thawan said about "fuzzy snapshots" makes me a little> > afraid that there is no simple solution.> >> >> > On Mon, Jul 8, 2013 at 11:05 PM, kishore g <[EMAIL PROTECTED]> wrote:> >> > > Agree, we already have such a tool. In fact we use it to reconstruct> the> > > sequence of events that led to a failure and actually restore the> system> > to> > > a previous stable point and replay the events. Unfortunately this is> tied> > > closely with Helix but it should be easy to make this a generic tool.> > >> > > Sergey is this something that will be useful in your case.> > >> > > Thanks,> > > Kishore G> > >> > >> > > On Mon, Jul 8, 2013 at 8:09 PM, Thawan Kooburat <[EMAIL PROTECTED]> wrote:> > >> > > > On restore part, I think having a separate utility to manipulate the> > > > data/snap dir (by truncating the log/removing snapshot to a given> zxid)> > > > would be easier than modifying the server.> > > >> > > >> > > > --> > > > Thawan Kooburat> > > >> > > >> > > >> > > >> > > >> > > > On 7/8/13 6:34 PM, "kishore g" <[EMAIL PROTECTED]> wrote:> > > >> > > > >I think what we are looking at is a point in time restore> > > functionality.> > > > >How about adding a feature that says go back to a specific> > > zxid/timestamp.> > > > >This way before doing any change to zookeeper simply note down the> > > > >timestamp/zxid on leader. If things go wrong after making changes,> > bring> > > > >down zookeepers and provide additional parameter of a zxid/timestamp> > > while> > > > >restarting. The server can go the exact point and make it current.> The> > > > >followers can be started blank.> > > > >> > > > >> > > > >> > > > >On Mon, Jul 8, 2013 at 5:53 PM, Thawan Kooburat <[EMAIL PROTECTED]>> > wrote:> > > > >> > > > >> Just saw that this is the corresponding use case to the question> > > posted> > > > >> in dev list.> > > > >>> > > > >> In order to restore the data to a given point in time correctly,> you> > > > >>need> > > > >> both snapshot and txnlog. This is because zookeeper snapshot is

The snapshot will include any or all of those 5 updates. But the logs fromthat time *will* include all 5.

Thus, if you apply that part of the log to the snapshot, you will eitheroverwrite the latest value (with no effect) or you will update the previousvalue to the latest value for each of those 5 log entries. There areobviously 1024 possible alternatives here, but they all lead to the samefinal state and that is exactly a moment in time snapshot as of the finaltransaction.

The fuzzy term refers to the fact that you can't say exactly what time thethe original snapshot corresponds to. In your example, the data in thesnapshot represents a combination of states from anywhere in the 30 secondwindow that it took to write the snapshot.

> I think I am having difficulties understanding the "fuzzy" concept. Let's> say I started to serialize DataTree into a snapshot file and it took 30> seconds. During these 30 seconds, the server saw 5 transactions that> updated the data. Does this mean that the snapshot that I get on disk at> the end of the 30-second interval will have some of these 5 transactions?> Or will it have none? Or will it have all of them? Or will it be> inconsistent and unreadable by Zookeeper?>> Please help me better understand the behavior behind the "fuzzy" term.>> For my use case, I am perfectly fine if I get a snapshot with none of these> 5 transactions, considering that I will pick them up next time I take a> snapshot.>>> /Sergey>>> On Tue, Jul 9, 2013 at 12:08 AM, kishore g <[EMAIL PROTECTED]> wrote:>> > Its not really elaborate, it is very similar to what zookeeper does when> it> > starts up. It first reads the latest snapshot file and then the> transaction> > logs and applies each and every transaction. What I am suggesting is that> > instead of applying all transactions stop at a transaction i provide.> >> > Having this tool will actually simplify your task, you can go back to any> > point in time. Think of a something like this.> >> > checkpoint A // this can store the last zxid or timestamp from the> leader.> > Make changes to zk> > //if things fails> > stop zks> > rollback A//run this on each zk, brings back the cluster to its previous> > state.> > start zks // any order should be fine.> >> >> > Also keep in mind that snapshot is fuzzy only if there are writes> happening> > while taking snapshot. If you are sure no writes will happen when you are> > taking the snapshot then you are good. Experts, please correct me if this> > is incorrect.> >> > thanks,> > Kishore G> >> >> > On Mon, Jul 8, 2013 at 9:42 PM, Sergey Maslyakov <[EMAIL PROTECTED]>> > wrote:> >> > > Kishore,> > >> > > This sounds like a very elaborate tool. I was trying to find a> simplistic> > > approach but what Thawan said about "fuzzy snapshots" makes me a little> > > afraid that there is no simple solution.> > >> > >> > > On Mon, Jul 8, 2013 at 11:05 PM, kishore g <[EMAIL PROTECTED]>> wrote:> > >> > > > Agree, we already have such a tool. In fact we use it to reconstruct> > the> > > > sequence of events that led to a failure and actually restore the> > system> > > to> > > > a previous stable point and replay the events. Unfortunately this is> > tied> > > > closely with Helix but it should be easy to make this a generic tool.> > > >> > > > Sergey is this something that will be useful in your case.> > > >> > > > Thanks,> > > > Kishore G> > > >> > > >> > > > On Mon, Jul 8, 2013 at 8:09 PM, Thawan Kooburat <[EMAIL PROTECTED]>> wrote:> > > >> > > > > On restore part, I think having a separate utility to manipulate> the> > > > > data/snap dir (by truncating the log/removing snapshot to a given> > zxid)> > > > > would be easier than modifying the server.> > > > >> > > > >> > > > > --> > > > > Thawan Kooburat> > > > >> > > > >> > > > >> > > > >> > > > >> > > > > On 7/8/13 6:34 PM, "kishore g" <[EMAIL PROTECTED]> wrote:

The snapshot might have some of those transactions, it depends on when itreads the znode affected by the transaction. Say you have txn T that setsthe data of /a. When generating the snapshot, if it serializes /a before Tis committed, then the snapshot will not include T. Otw, it includes T.

I think I am having difficulties understanding the "fuzzy" concept. Let'ssay I started to serialize DataTree into a snapshot file and it took 30seconds. During these 30 seconds, the server saw 5 transactions that updatedthe data. Does this mean that the snapshot that I get on disk at the end ofthe 30-second interval will have some of these 5 transactions?Or will it have none? Or will it have all of them? Or will it beinconsistent and unreadable by Zookeeper?

For my use case, I am perfectly fine if I get a snapshot with none of these5 transactions, considering that I will pick them up next time I take asnapshot./SergeyOn Tue, Jul 9, 2013 at 12:08 AM, kishore g <[EMAIL PROTECTED]> wrote:

> Its not really elaborate, it is very similar to what zookeeper does > when it starts up. It first reads the latest snapshot file and then > the transaction logs and applies each and every transaction. What I am > suggesting is that instead of applying all transactions stop at atransaction i provide.>> Having this tool will actually simplify your task, you can go back to > any point in time. Think of a something like this.>> checkpoint A // this can store the last zxid or timestamp from the leader.> Make changes to zk> //if things fails> stop zks> rollback A//run this on each zk, brings back the cluster to its > previous state.> start zks // any order should be fine.>>> Also keep in mind that snapshot is fuzzy only if there are writes > happening while taking snapshot. If you are sure no writes will happen > when you are taking the snapshot then you are good. Experts, please > correct me if this is incorrect.>> thanks,> Kishore G>>> On Mon, Jul 8, 2013 at 9:42 PM, Sergey Maslyakov <[EMAIL PROTECTED]>> wrote:>> > Kishore,> >> > This sounds like a very elaborate tool. I was trying to find a > > simplistic approach but what Thawan said about "fuzzy snapshots" > > makes me a little afraid that there is no simple solution.> >> >> > On Mon, Jul 8, 2013 at 11:05 PM, kishore g <[EMAIL PROTECTED]> wrote:> >> > > Agree, we already have such a tool. In fact we use it to > > > reconstruct> the> > > sequence of events that led to a failure and actually restore the> system> > to> > > a previous stable point and replay the events. Unfortunately this > > > is> tied> > > closely with Helix but it should be easy to make this a generic tool.> > >> > > Sergey is this something that will be useful in your case.> > >> > > Thanks,> > > Kishore G> > >> > >> > > On Mon, Jul 8, 2013 at 8:09 PM, Thawan Kooburat <[EMAIL PROTECTED]> wrote:> > >> > > > On restore part, I think having a separate utility to manipulate > > > > the data/snap dir (by truncating the log/removing snapshot to a > > > > given> zxid)> > > > would be easier than modifying the server.> > > >> > > >> > > > --> > > > Thawan Kooburat> > > >> > > >> > > >> > > >> > > >> > > > On 7/8/13 6:34 PM, "kishore g" <[EMAIL PROTECTED]> wrote:> > > >> > > > >I think what we are looking at is a point in time restore> > > functionality.> > > > >How about adding a feature that says go back to a specific> > > zxid/timestamp.> > > > >This way before doing any change to zookeeper simply note down > > > > >the timestamp/zxid on leader. If things go wrong after making > > > > >changes,> > bring> > > > >down zookeepers and provide additional parameter of a

Sorry Flavio, I mixed two things in my previous email. When i saidcheckpoint A, it means just save the last committed transaction id (Nosnapshot will be taken). When we need to do restore we will simply run thetool to bring the data directory to that particular zxid( We will truncatethe txn log after that zxid). We can now restart the server and we shouldget back to that particular point.The second part about fuzzy snapshot, I was just trying to explain toSergey that its not really fuzzy if he knows for sure that there are noupdates while taking snapshot. This really depends on the use case, forexample if all writes happen via a manually run tool then snapshot shouldnot be fuzzy.

> I think I am having difficulties understanding the "fuzzy" concept. Let's> say I started to serialize DataTree into a snapshot file and it took 30> seconds. During these 30 seconds, the server saw 5 transactions that> updated the data. Does this mean that the snapshot that I get on disk at> the end of the 30-second interval will have some of these 5 transactions?> Or will it have none? Or will it have all of them? Or will it be> inconsistent and unreadable by Zookeeper?>> Please help me better understand the behavior behind the "fuzzy" term.>> For my use case, I am perfectly fine if I get a snapshot with none of these> 5 transactions, considering that I will pick them up next time I take a> snapshot.>>> /Sergey>>> On Tue, Jul 9, 2013 at 12:08 AM, kishore g <[EMAIL PROTECTED]> wrote:>> > Its not really elaborate, it is very similar to what zookeeper does when> it> > starts up. It first reads the latest snapshot file and then the> transaction> > logs and applies each and every transaction. What I am suggesting is that> > instead of applying all transactions stop at a transaction i provide.> >> > Having this tool will actually simplify your task, you can go back to any> > point in time. Think of a something like this.> >> > checkpoint A // this can store the last zxid or timestamp from the> leader.> > Make changes to zk> > //if things fails> > stop zks> > rollback A//run this on each zk, brings back the cluster to its previous> > state.> > start zks // any order should be fine.> >> >> > Also keep in mind that snapshot is fuzzy only if there are writes> happening> > while taking snapshot. If you are sure no writes will happen when you are> > taking the snapshot then you are good. Experts, please correct me if this> > is incorrect.> >> > thanks,> > Kishore G> >> >> > On Mon, Jul 8, 2013 at 9:42 PM, Sergey Maslyakov <[EMAIL PROTECTED]>> > wrote:> >> > > Kishore,> > >> > > This sounds like a very elaborate tool. I was trying to find a> simplistic> > > approach but what Thawan said about "fuzzy snapshots" makes me a little> > > afraid that there is no simple solution.> > >> > >> > > On Mon, Jul 8, 2013 at 11:05 PM, kishore g <[EMAIL PROTECTED]>> wrote:> > >> > > > Agree, we already have such a tool. In fact we use it to reconstruct> > the> > > > sequence of events that led to a failure and actually restore the> > system> > > to> > > > a previous stable point and replay the events. Unfortunately this is> > tied> > > > closely with Helix but it should be easy to make this a generic tool.> > > >> > > > Sergey is this something that will be useful in your case.> > > >> > > > Thanks,> > > > Kishore G> > > >> > > >> > > > On Mon, Jul 8, 2013 at 8:09 PM, Thawan Kooburat <[EMAIL PROTECTED]>> wrote:> > > >> > > > > On restore part, I think having a separate utility to manipulate> the> > > > > data/snap dir (by truncating the log/removing snapshot to a given> > zxid)> > > > > would be easier than modifying the server.> > > > >> > > > >> > > > > --> > > > > Thawan Kooburat> > > > >> > > > >> > > > >> > > > >> > > > >> > > > > On 7/8/13 6:34 PM, "kishore g" <[EMAIL PROTECTED]> wrote:

Sorry Flavio, I mixed two things in my previous email. When i saidcheckpoint A, it means just save the last committed transaction id (Nosnapshot will be taken). When we need to do restore we will simply run thetool to bring the data directory to that particular zxid( We will truncatethe txn log after that zxid). We can now restart the server and we shouldget back to that particular point.The second part about fuzzy snapshot, I was just trying to explain to Sergeythat its not really fuzzy if he knows for sure that there are no updateswhile taking snapshot. This really depends on the use case, for example ifall writes happen via a manually run tool then snapshot should not be fuzzy.

> I think I am having difficulties understanding the "fuzzy" concept. > Let's say I started to serialize DataTree into a snapshot file and it > took 30 seconds. During these 30 seconds, the server saw 5 > transactions that updated the data. Does this mean that the snapshot > that I get on disk at the end of the 30-second interval will have some ofthese 5 transactions?> Or will it have none? Or will it have all of them? Or will it be > inconsistent and unreadable by Zookeeper?>> Please help me better understand the behavior behind the "fuzzy" term.>> For my use case, I am perfectly fine if I get a snapshot with none of > these> 5 transactions, considering that I will pick them up next time I take > a snapshot.>>> /Sergey>>> On Tue, Jul 9, 2013 at 12:08 AM, kishore g <[EMAIL PROTECTED]> wrote:>> > Its not really elaborate, it is very similar to what zookeeper does > > when> it> > starts up. It first reads the latest snapshot file and then the> transaction> > logs and applies each and every transaction. What I am suggesting is > > that instead of applying all transactions stop at a transaction iprovide.> >> > Having this tool will actually simplify your task, you can go back > > to any point in time. Think of a something like this.> >> > checkpoint A // this can store the last zxid or timestamp from the> leader.> > Make changes to zk> > //if things fails> > stop zks> > rollback A//run this on each zk, brings back the cluster to its > > previous state.> > start zks // any order should be fine.> >> >> > Also keep in mind that snapshot is fuzzy only if there are writes> happening> > while taking snapshot. If you are sure no writes will happen when > > you are taking the snapshot then you are good. Experts, please > > correct me if this is incorrect.> >> > thanks,> > Kishore G> >> >> > On Mon, Jul 8, 2013 at 9:42 PM, Sergey Maslyakov <[EMAIL PROTECTED]>> > wrote:> >> > > Kishore,> > >> > > This sounds like a very elaborate tool. I was trying to find a> simplistic> > > approach but what Thawan said about "fuzzy snapshots" makes me a > > > little afraid that there is no simple solution.> > >> > >> > > On Mon, Jul 8, 2013 at 11:05 PM, kishore g <[EMAIL PROTECTED]>> wrote:> > >> > > > Agree, we already have such a tool. In fact we use it to > > > > reconstruct> > the> > > > sequence of events that led to a failure and actually restore > > > > the> > system> > > to> > > > a previous stable point and replay the events. Unfortunately > > > > this is> > tied> > > > closely with Helix but it should be easy to make this a generictool.> > > >> > > > Sergey is this something that will be useful in your case.> > > >> > > > Thanks,> > > > Kishore G> > > >> > > >> > > > On Mon, Jul 8, 2013 at 8:09 PM, Thawan Kooburat <[EMAIL PROTECTED]>> wrote:> > > >> > > > > On restore part, I think having a separate utility to current.

Restoring a single server is not a big deal, but restoring data in anensemble gets tricky when there is no mechanism to "initialize" theensemble data store to a certain state. What I came up with so far is justa couple of work-arounds that trick Zookeeper logic to follow the path Icannot directly enforce.On Mon, Jul 8, 2013 at 10:09 PM, Thawan Kooburat <[EMAIL PROTECTED]> wrote:

> On restore part, I think having a separate utility to manipulate the> data/snap dir (by truncating the log/removing snapshot to a given zxid)> would be easier than modifying the server.>>> --> Thawan Kooburat>>>>>> On 7/8/13 6:34 PM, "kishore g" <[EMAIL PROTECTED]> wrote:>> >I think what we are looking at is a point in time restore functionality.> >How about adding a feature that says go back to a specific zxid/timestamp.> >This way before doing any change to zookeeper simply note down the> >timestamp/zxid on leader. If things go wrong after making changes, bring> >down zookeepers and provide additional parameter of a zxid/timestamp while> >restarting. The server can go the exact point and make it current. The> >followers can be started blank.> >> >> >> >On Mon, Jul 8, 2013 at 5:53 PM, Thawan Kooburat <[EMAIL PROTECTED]> wrote:> >> >> Just saw that this is the corresponding use case to the question posted> >> in dev list.> >>> >> In order to restore the data to a given point in time correctly, you> >>need> >> both snapshot and txnlog. This is because zookeeper snapshot is fuzzy> >>and> >> snapshot alone may not represent a valid state of the server if there> >>are> >> in-flight requests.> >>> >> The 4wl command should cause the server to roll the log and take a> >> snapshot similar to periodic snapshotting operation. Your backup script> >> need grap the snapshot and corresponding txnlog file from the data dir.> >>> >> To restore, just shutdown all hosts, clear the data dir, copy over the> >> snapshot and txnlog, and restart them.> >>> >>> >> --> >> Thawan Kooburat> >>> >>> >>> >>> >>> >> On 7/8/13 3:28 PM, "Sergey Maslyakov" <[EMAIL PROTECTED]> wrote:> >>> >> >Thank you for your response, Flavio. I apologize, I did not provide a> >> >clear> >> >explanation of the use case.> >> >> >> >This backup/restore is not intended to be tied to any write event,> >> >instead,> >> >it is expected to run as a periodic (daily?) cron job on one of the> >> >servers, which is not guaranteed to be the leader of the ensemble.> >>There> >> >is> >> >no expectation that all recent changes are committed and persisted to> >> >disk.> >> >The system can sustain the loss of several hours worth of recent> >>changes> >> >in> >> >the event of restore.> >> >> >> >As for finding the leader dynamically and performing backup on it, this> >> >approach could be more difficult as the leader can change time to time> >>and> >> >I still need to fetch the file to store it in my designated backup> >> >location. Taking backup on one server and picking it up from a local> >>file> >> >system looks less error-prone. Even if I went the fancy route and had> >> >Zookeeper send me the serialized DataTree in response to the 4wl, this> >> >approach would involve a lot of moving parts.> >> >> >> >I have already made a PoC for a new 4wl that invokes takeSnapshot() and> >> >returns an absolute path to the snapshot it drops on disk. I have> >>already> >> >protected takeSnapshot() from concurrent invocation, which is likely to> >> >corrupt the snapshot file on disk. This approach works but I'm> >>thinking to> >> >take it one step further by providing the desired path name as an> >>argument> >> >to my new 4lw and to have Zookeeper server drop the snapshot into the> >> >specified file and report success/failure back. This way I can avoid> >> >cluttering the data directory and interfering with what Zookeeper finds> >> >when it scans the data directory.> >> >> >> >Approach with having an additional server that would take the

> I think what we are looking at is a point in time restore functionality.> How about adding a feature that says go back to a specific zxid/timestamp.> This way before doing any change to zookeeper simply note down the> timestamp/zxid on leader. If things go wrong after making changes, bring> down zookeepers and provide additional parameter of a zxid/timestamp while> restarting. The server can go the exact point and make it current. The> followers can be started blank.>>>> On Mon, Jul 8, 2013 at 5:53 PM, Thawan Kooburat <[EMAIL PROTECTED]> wrote:>> > Just saw that this is the corresponding use case to the question posted> > in dev list.> >> > In order to restore the data to a given point in time correctly, you need> > both snapshot and txnlog. This is because zookeeper snapshot is fuzzy and> > snapshot alone may not represent a valid state of the server if there are> > in-flight requests.> >> > The 4wl command should cause the server to roll the log and take a> > snapshot similar to periodic snapshotting operation. Your backup script> > need grap the snapshot and corresponding txnlog file from the data dir.> >> > To restore, just shutdown all hosts, clear the data dir, copy over the> > snapshot and txnlog, and restart them.> >> >> > --> > Thawan Kooburat> >> >> >> >> >> > On 7/8/13 3:28 PM, "Sergey Maslyakov" <[EMAIL PROTECTED]> wrote:> >> > >Thank you for your response, Flavio. I apologize, I did not provide a> > >clear> > >explanation of the use case.> > >> > >This backup/restore is not intended to be tied to any write event,> > >instead,> > >it is expected to run as a periodic (daily?) cron job on one of the> > >servers, which is not guaranteed to be the leader of the ensemble. There> > >is> > >no expectation that all recent changes are committed and persisted to> > >disk.> > >The system can sustain the loss of several hours worth of recent changes> > >in> > >the event of restore.> > >> > >As for finding the leader dynamically and performing backup on it, this> > >approach could be more difficult as the leader can change time to time> and> > >I still need to fetch the file to store it in my designated backup> > >location. Taking backup on one server and picking it up from a local> file> > >system looks less error-prone. Even if I went the fancy route and had> > >Zookeeper send me the serialized DataTree in response to the 4wl, this> > >approach would involve a lot of moving parts.> > >> > >I have already made a PoC for a new 4wl that invokes takeSnapshot() and> > >returns an absolute path to the snapshot it drops on disk. I have> already> > >protected takeSnapshot() from concurrent invocation, which is likely to> > >corrupt the snapshot file on disk. This approach works but I'm thinking> to> > >take it one step further by providing the desired path name as an> argument> > >to my new 4lw and to have Zookeeper server drop the snapshot into the> > >specified file and report success/failure back. This way I can avoid> > >cluttering the data directory and interfering with what Zookeeper finds> > >when it scans the data directory.> > >> > >Approach with having an additional server that would take the leadership> > >and populate the ensemble is just a theory. I don't see a clean way of> > >making a quorum member the leader of the quorum. Am I overlooking> > >something> > >simple?> > >> > >In backup and restore of an ensemble the biggest unknown for me remains> > >populating the ensemble with desired data. I can think of two ways:> > >> > >1. Clear out all servers by stopping them, purge version-2 directories,> > >restore a snapshot file on one server that will be brought first, and> then> > >bring up the rest of the ensemble. This way I somewhat force the first> > >server to be the leader because it has data and it will be the only

These are interesting points, Thawan. I'd like to make sure that I get themright.

1. Are you saying that a snapshot file may not be sufficient to restoreZookeeper to a consistent state? Does it always require a transaction logfile or is it required to get to the most current state? I was hoping thata snapshot is self-sufficient to do a restore to recent but not necessarilymost current state. Was I wrong?

2. Do you suggest that the same pair of a snapshot (and a transaction log)needs to be copied on all servers before they are brought online? The whatabout the "epoch" files? Do they need to be purged, preserved, or same onepopulated through the whole ensemble?On Mon, Jul 8, 2013 at 7:53 PM, Thawan Kooburat <[EMAIL PROTECTED]> wrote:

> Just saw that this is the corresponding use case to the question posted> in dev list.>> In order to restore the data to a given point in time correctly, you need> both snapshot and txnlog. This is because zookeeper snapshot is fuzzy and> snapshot alone may not represent a valid state of the server if there are> in-flight requests.>> The 4wl command should cause the server to roll the log and take a> snapshot similar to periodic snapshotting operation. Your backup script> need grap the snapshot and corresponding txnlog file from the data dir.>> To restore, just shutdown all hosts, clear the data dir, copy over the> snapshot and txnlog, and restart them.>>> --> Thawan Kooburat>>>>>> On 7/8/13 3:28 PM, "Sergey Maslyakov" <[EMAIL PROTECTED]> wrote:>> >Thank you for your response, Flavio. I apologize, I did not provide a> >clear> >explanation of the use case.> >> >This backup/restore is not intended to be tied to any write event,> >instead,> >it is expected to run as a periodic (daily?) cron job on one of the> >servers, which is not guaranteed to be the leader of the ensemble. There> >is> >no expectation that all recent changes are committed and persisted to> >disk.> >The system can sustain the loss of several hours worth of recent changes> >in> >the event of restore.> >> >As for finding the leader dynamically and performing backup on it, this> >approach could be more difficult as the leader can change time to time and> >I still need to fetch the file to store it in my designated backup> >location. Taking backup on one server and picking it up from a local file> >system looks less error-prone. Even if I went the fancy route and had> >Zookeeper send me the serialized DataTree in response to the 4wl, this> >approach would involve a lot of moving parts.> >> >I have already made a PoC for a new 4wl that invokes takeSnapshot() and> >returns an absolute path to the snapshot it drops on disk. I have already> >protected takeSnapshot() from concurrent invocation, which is likely to> >corrupt the snapshot file on disk. This approach works but I'm thinking to> >take it one step further by providing the desired path name as an argument> >to my new 4lw and to have Zookeeper server drop the snapshot into the> >specified file and report success/failure back. This way I can avoid> >cluttering the data directory and interfering with what Zookeeper finds> >when it scans the data directory.> >> >Approach with having an additional server that would take the leadership> >and populate the ensemble is just a theory. I don't see a clean way of> >making a quorum member the leader of the quorum. Am I overlooking> >something> >simple?> >> >In backup and restore of an ensemble the biggest unknown for me remains> >populating the ensemble with desired data. I can think of two ways:> >> >1. Clear out all servers by stopping them, purge version-2 directories,> >restore a snapshot file on one server that will be brought first, and then> >bring up the rest of the ensemble. This way I somewhat force the first> >server to be the leader because it has data and it will be the only member> >of a quorum with data, provided to the way I start the ensemble. This

I want to make sure I am fully understanding the procedures of zookeeperbackup and disaster recovery:

For the backup procedures at zookeeper assemble:(1) Login to any host which state is "Serving" Question: Do I have to login to leader node, or any node is ok?(2) Copy latest snapshot file and transaction log from version-2 directory. Question: How to make sure we do not copy corrupt files if thesnapshot/transaction log is in the middle of update? Do we have to shutdownthe node to make the copy? besides the transaction log and snapshot, do we have tocopy other files such as the ecoch files

For the disaster recovery procedures at zookeeper assemble:(1) recreate the machines for the zookeeper ensemble(2) copy snapshot/transaction log we backed up into the zookeeperdataDir\version-2 and logDir\version2. Question: Do we have to copy the epoch files? Do we have to copy snapshot/transaction log backed up toall the zookeeper node, or just the first node we starts?

> These are interesting points, Thawan. I'd like to make sure that I get them> right.>> 1. Are you saying that a snapshot file may not be sufficient to restore> Zookeeper to a consistent state? Does it always require a transaction log> file or is it required to get to the most current state? I was hoping that> a snapshot is self-sufficient to do a restore to recent but not necessarily> most current state. Was I wrong?>> 2. Do you suggest that the same pair of a snapshot (and a transaction log)> needs to be copied on all servers before they are brought online? The what> about the "epoch" files? Do they need to be purged, preserved, or same one> populated through the whole ensemble?>>> On Mon, Jul 8, 2013 at 7:53 PM, Thawan Kooburat <[EMAIL PROTECTED]> wrote:>> > Just saw that this is the corresponding use case to the question posted> > in dev list.> >> > In order to restore the data to a given point in time correctly, you need> > both snapshot and txnlog. This is because zookeeper snapshot is fuzzy and> > snapshot alone may not represent a valid state of the server if there are> > in-flight requests.> >> > The 4wl command should cause the server to roll the log and take a> > snapshot similar to periodic snapshotting operation. Your backup script> > need grap the snapshot and corresponding txnlog file from the data dir.> >> > To restore, just shutdown all hosts, clear the data dir, copy over the> > snapshot and txnlog, and restart them.> >> >> > --> > Thawan Kooburat> >> >> >> >> >> > On 7/8/13 3:28 PM, "Sergey Maslyakov" <[EMAIL PROTECTED]> wrote:> >> > >Thank you for your response, Flavio. I apologize, I did not provide a> > >clear> > >explanation of the use case.> > >> > >This backup/restore is not intended to be tied to any write event,> > >instead,> > >it is expected to run as a periodic (daily?) cron job on one of the> > >servers, which is not guaranteed to be the leader of the ensemble. There> > >is> > >no expectation that all recent changes are committed and persisted to> > >disk.> > >The system can sustain the loss of several hours worth of recent changes> > >in> > >the event of restore.> > >> > >As for finding the leader dynamically and performing backup on it, this> > >approach could be more difficult as the leader can change time to time> and> > >I still need to fetch the file to store it in my designated backup> > >location. Taking backup on one server and picking it up from a local> file> > >system looks less error-prone. Even if I went the fancy route and had> > >Zookeeper send me the serialized DataTree in response to the 4wl, this> > >approach would involve a lot of moving parts.> > >> > >I have already made a PoC for a new 4wl that invokes takeSnapshot() and

+

jack ma 2013-07-16, 15:38

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